1. Technical Field
The present invention concerns improvements in or relating to hearing aids and more particularly hearing aids such as for example "in-the-canal" (ITC), "in-the-ear" (ITE), and "behind the ear" (BTE) hearing aids which share the common feature that the microphone and receiver components thereof are mounted on a single body and are closely spaced.
A number of mechanisms can contribute to the acoustic feedback that occurs in these aids. In all cases there will occur an unavoidable acoustic or mechanical coupling through the body or the inner volume of the hearing aid. Acoustic feedback will also occur as sound is directed by reflection and is channelled from the receiver to the microphone. Thus in the case of "behind-the-ear" aids sound can be reflected by the surrounding head structures and in the case of "in-the-canal" and "in-the-ear aids" acoustic coupling can occur through an unintended aperture around the periphery of the mould of the hearing aid.
In the case of "in-the-ear" hearing aids, venting is included as a means for reducing occlusion--that is to say, to avoid a blocked ear sensation. A further benefit of including venting is that it allows the direct input of unamplified low frequency sound into the ear--an important benefit for those patients that suffer a high frequency loss with only minor loss at low frequency and who have therefore no need for amplification in this lower frequency range. There is also available a wide range of vent sizes. Whilst larger diameter vents are more effective for reducing occlusion than vents of smaller diameter, they also give rise to an increase in acoustic feedback. Vent sizes are thus limited by the attendant need to avoid excessive acoustic feedback. Large sizes of vent now are used infrequently and are limited in application to aids using only lower gain amplification.
A number of problems are associated with acoustic feedback. The most commonly considered is the occurrence of oscillation. This occurs at high gain level and is usually persistent. At gain settings just below those that cause such oscillation "ringing effects" are perceivable. This latter causes further unpleasant sensation and reduces the intelligability of speech and other wanted sound. An additional effect is that the acoustic feedback contributes to gain dependant perturbations in the frequency response of the hearing aid. Such uncontrolled frequency response effects occur at gains well below the onset of oscillation and can cause quite discernable, and sometimes considerable, distortion in the perceived sound, and accordingly do reduce the effectiveness of the hearing aid in meeting prescribed characteristics.
2. Reference to Prior Art
It is difficult to avoid the occurrence of acoustic feedback. Thus in the case of "in-the-ear" aids leakage can be reduced by making the ear mould a tighter fit. This however is hard to achieve and is far from ideally comfortable for the user. It is also not possible to eliminate internal acoustic feedback. In both cases however it is possible to reduce the level of acoustic feedback by careful positioning of the components, particularly the direction and position of the microphone port.
Since the elimination of acoustic feedback is not possible, effort has been directed to suppress the occurrence of oscillation. The frequency response of hearing aids means that oscillation usually occurs at frequencies in the range 1 to 4 kHz and the exact frequency is set by the normal Nyquist conditions (i.e. loop gain that is greater than zero decibels and zero phase difference between wanted signal and echo).
Oscillatory conditions may be suppressed by the user turning down the volume control, i.e. reducing gain, once oscillation has occurred. This however is generally inconvenient to the user. Therefore techniques have been developed for preventing the occurrence of oscillation. Thus the electronic gain of an aid has been reduced for frequencies where oscillation is likely to occur. This however means that the prescribed high frequency response for a deaf user would be compromised deliberately at these high frequencies which are usually in the range 1 to 4 kHz and also quite near to the natural resonant frequency of the ear canal of about 3 kHz. Also receivers have been designed to reduce their electro-acoustic gain particularly at the resonant frequency of the receiver where a relative gain figure of 10 decibels or more can occur. However, this peak is normally desirable so as to reproduce the natural canal resonances of a normal ear.
In all cases of oscillation avoidance considered above it will be noted that a design compromise is necessary and that in consequence such hearing aids can only offer a sub-optimum performance to the user.
Other corrective techniques have involved the detection of the onset of oscillation and the provision of cancellation or other compensation in response. Thus a detected signal has been used as a means of controlling an automatic gain stage. An alternative technique is to use the detected signal to control the generation of an internal oscillation and to use the latter to cancel the unwanted oscillation. This technique has to be adaptive since the oscillation frequency and amplitude will vary according to changes in external conditions. Furthermore the detected signal has been used to adjust the centre frequency of a notch filter, that is to say a filter with a frequency selective reduction in gain centered at or near the oscillation frequency.
In the above techniques involving oscillation detection however no compensation is provided for the effects of acoustic feedback at those frequencies remote from the oscillation frequency.
An alternative approach to the above is proposed in U.S. Pat. No. 4,783,818. As proposed the effects of acoustic feedback are compensated by electrical negative feedback. The negative feedback path includes a filter the characteristics of which are modeled on those identified inter alia for the acoustic feedback path itself. The communication system described, which may be for example a hearing aid, is constructed to have two time consecutive modes of operation: an ordinary operational mode; and an identification mode. In the ordinary operational mode the compensated electrical signal is fed through an amplifier and thereon used to energise the receiver. In the identification mode, which may be selected e.g. at turn-on or in response to an automatically sensed threshhold change in amplifier gain, the amplifier is decoupled from the circuit and a correction circuit is substituted. The correction circuit includes a source of noise for example pseudo-random-binary-sequence signals and parameters associated only with acoustic feedback are identified and used to define the transfer function of the filter that is then subsequently used during ordinary operation. It is a disadvantage of this construction however that during the identification mode of operation the hearing aid behaves as a generator of acoustic noise and does not provide any effective hearing aid operation at all. Also ordinary operation is only satisfactory so long as the acoustic feedback remains substantially the same as that for which the parameters are identified. The proposed construction therefore is not entirely satisfactory for hearing aid application.
This type of approach has also been considered experimentally. See the following thesis entitled "Digital Suppression of Acoustic Feedback in Hearing Aids" by Leland C. Best, University of Wyoming, May 1985. In this are described several experiments on an adaptive digital filter applied as a continuously adaptive feedback path to compensate for acoustic feedback in the hearing aid. A digital random number generator using the congruence method is employed as the source of noise which is injected into the acoustic path for deriving adaptively the filter parameters.
The experimental apparatus described in this thesis does not form a practical hearing aid. This is principally because of the problem of non-linearities in the feedback, as described in Chapters IV and VI of this reference. Such non-linearities become manifest when the output signal momentarily exceeds the linear region of the response of the output transducer. Under these circumstances the digital filter is no longer able to produce an accurate replica cancelling the feedback signals. Since the digital filter would generally be employed to enhance the gain of the hearing and, it would be quite common for both the acoustic path and the digital filter path each to be unstable when separate, only stabilising each other by producing accurate replicas of each others feedback signals. At the onset of non-linearity as described the feedback signals in the two parts no longer cancel each other and both parts become unstable. The hearing aid therefore becomes unstable, and may remain so indefinitely.
In Chapter IV of this reference, this problem is solved for experimental purposes by detecting that this situation is likely to have occurred, and silencing the transmission of output signal until this system recovers. This is performed for example by ramping down the amplifier gain to zero. See pages 26 to 28 of this reference. Such a solution would not be acceptable in practice however since it would result in fairly frequent interruptions of the amplified sound.